This invention relates to marine seismic surveying and, more particularly, to systems for enhancing marine seismic profiles to minimize destructive effects of water column reverberation.
Generally speaking, marine seismic surveys are conducted by towing an energy source and seismic detectors behind a vessel. The source imparts an acoustic wave to the water, creating a wavefield which travels coherently into the underlying earth. As the wavefield strikes interfaces between earth formations, or strata, it is reflected back through the earth and water to the detectors, where it is converted to electrical signals and recorded. Through analysis of these signals, it is possible to determine the shape, position and lithology of the sub-bottom formations.
A problem encountered in marine surveying--as well as in inverse vertical seismic profiling, or "VSP"--is that of water column reverberation. The problem, which arises as a result of the inherent reflectivity of the water surface and bottom, may be explained as follows.
A seismic wave generated in (or reflected off) earth strata passes into the water in a generally upward direction. This wave, termed the "primary," travels through the water and past the seismic detector which records its presence. The wavefield continues upward to the water's surface, where it is reflected back downwards. This reflected, or "ghost," wavefield also travels through the water and past the detector, where it is again recorded. Depending upon the nature of the earth material at the water's bottom, the ghost wavefield may itself be reflected upwards through the water, giving rise to a series of one or more subsequent ghost reflections.
This reverberation of the seismic wavefield in the water obscures seismic data, amplifying certain frequencies and attenuating others, thereby making it difficult to analyze the underlying earth formations.
In instances where the earth material at the water bottom is particularly hard, excess acoustic energy or noise generated by the seismic source can also become trapped in the water column, reverberating in the same manner as the reflected seismic waves themselves. This noise is often high in amplitude and, as a result, tends to cover the weaker seismic reflection signals sought for study.
In the art, Ruehle, U.S. Pat. No. 4,486,865, discloses a technique for reducing ghosting wherein a pressure detector and a particle velocity detector are positioned in close proximity to one another in the water. The output of at least one of the detectors is gain-adjusted and filtered, using a deconvolution operation having a predetermined amount of white noise to the zero lag of the autocorrelation function. The patent suggests that, by adding this deconvolved/gain-adjusted signal to the output of the other detector, ghost reflections may be cancelled.
Haggerty, U.S. Pat. No. 2,757,356, discloses a marine seismic reflection surveying system in which two seismometer spreads are disposed at two distinct depths in the water such that water column reverberations received by them are 180 degrees out of phase. By combining the output of the detectors, the patent suggests that the reverberations will cancel.
Berni, U.S. Pat. No. 4,345,473, suggests the use of a vertical component accelerometer in combination with a hydrophone for cancelling surface-reflected noise in marine seismic operations.
Gal'perin, "Vertical Seismic Profiling," Special Publication No. 12 of the Society of Exploration Geophysicists, suggests the use of a seismic detector which combines the output of a pressure and velocity sensor for use in VSP surveying.
While the aforementioned prior art techniques may be effective under certain circumstances, none is considered adequate for use in water-bottom cable surveying in deep water operations, i.e., surveys in depths greater than 50-100 feet. Moreover, techniques of the type proposed in the Ruehle patent suffer losses in signal quality inherent to the filtering process, for example, amplification of noise in certain frequencies.
Accordingly, an object of this invention is to provide an improved system for marine seismic reflection prospecting.
More particularly, an object of the invention is to provide methods for marine seismic prospecting capable of attenuating coherent noise resulting, for example, from water column reverberation.
A further object of the invention is to provide a marine seismic reflection prospecting system effective in all depths of water, including that greater than 50-100 feet.